The present invention relates to a device for selectively retaining a leading edge of a stencil printing master onto a printing drum of a rotary stencil printing device.
A certain type of rotary stencil printing device which incorporates a hollow printing drum around which a printing stencil master is wrapped is becoming widely used. In such a stencil printing device, printing ink is supplied from within the drum through perforations which extend from its inside space to its outer surface to the radially inner surface of the stencil master wrapped around the drum, and some of this ink passes through multiple perforations in certain previously perforated parts of the stencil master to the radially outward surface of the stencil master, whence this ink is then able to be transferred to the surface of a sheet of printing paper pressed against said outer surface of the stencil master, as the printing drum with the stencil master wrapped around it is rotated.
In such a rotary stencil printing device, a means is required to be provided for retaining a leading edge of the stencil master along an outer surface portion of the printing drum and parallel with with the central axis thereof (hereinafter called "peripheral line" for the sake of convenience), so that the stencil master is wrapped around the printing drum with a proper tension being applied thereto and the stencil sheet adhered around the printing drum by a viscous ink does not shift relative to the printing drum during the rotary stencil printing process.
For use with currently manufactured conventional stencil printing devices, the leading edge of the stencil master is formed with a reinforced lug portion, made for example of cardboard, and this lug portion is formed with a plurality of perforated engaging holes therein, and along the peripheral line of the printing drum there are provided a number of engagement projections corresponding to these engaging holes, the cardboard lug portion thus being retained against the printing drum along said peripheral line with the engagement projections engaged into the engaging holes. A clamp strip is generally provided to mechanically press the cardboard lug portion mechanically to the printing drum along said peripheral line, or such a clamp strip may be omitted whne the engagement between the holes and the projections is formed to be of a non-return type.
These conventional constructions for clamping the leading edge of the stencil master to the printing drum operate generally satisfactorily. However, there is still required a delicate and tedious hand job of properly aligning the lug portion of the stencil master against the peripheral line of the printing drum in a proper position for the engagment projections to be properly engaged into the engaging holes, and this fatally obstructs automation of the process of mounting and demounting of a stencil master to a printing drum. Further, because the stencil master, with this form of stencil retaining system, is required to have the above mentioned reinforcing lug provided along its leading edge, it is not possible to utilize continous stencil master which is cut off from a roll of stencil master as and when required, but instead the stencil master sheets must be individually manufactured in advance with reinforcing lugs thereon, which is expensive. The use of a headless stencil master, which is typically a piece of plastic material of the thickness of a few tens of microns, without any particular construction for the leading edge thereof, is a very desirable object for implementation in a stencil printing device, and accordingly the prior art schemes for retaining a leading edge of a stencil master onto a printing drum of a rotary stencil printing device are seriously deficient.